This invention relates to the preparation of elastomeric, polyurethane compositions. More particularly, this invention relates to novel compositions for preparing elastomeric polyurethanes wherein the reagents employed to prepare the polyurethanes include a difunctional amine as the chain extender.
Elastomeric polyurethanes are conventionally prepared by reacting an isocyanate-terminated prepolymer (obtained by reacting a polyol having a functionality of two or higher and a stoichiometric excess of a diisocyanate compound) with an amine as the chain extending agent. The elastomer is believed to be a block copolymer containing both urethane and urea groups.
Up until recently the preferred chain extender was 4, 4'-methylene-bis (2-chloroaniline), which is referred to in the trade as "MOCA". MOCA imparts excellent mechanical strength properties to urethane elastomers, however, the use of this diamine has been severely restricted since regulations appeared in the Federal Register, Vol. 38, No. 144, July 27, 1973. These regulations classify MOCA as a carcinogen and require that strict precautions be exercised during the manufacture, handling and use of this compound. These restrictions have encouraged a search for less objectionable chain extending agents which impart the same level of desirable physical properties as MOCA. U.S. Pat. No. 3,920,617 teaches that sulfur-containing diamines of the general formula ##STR1## wherein R is alkylene and R' is hydrogen, halogen or a hydrocarbon group are useful chain extending agents for urethane type elastomers, however, the rate at which this class of compounds achieves hardness is considerably slower than the rate achieved using other prior art amine-type chain extending agents such as MOCA. This is a considerable disadvantage in a commercial molding operation, since it increases the time required for the material to become sufficiently cured to allow it to be removed from the mold without permanent deformation or loss of structural integrity. The uncured mixture of isocyanate prepolymer and amine curing agent is usually a liquid of low to moderate viscosity. This liquid is poured into a heated mold wherein it is gradually converted to a solid, crosslinked material. The time interval between filling of the mold and the earliest time at which the object will retain its shape when removed from the mold is known as the "demold time". It is highly desirable to achieve the shortest possible demold time, since this will permit an increase in production rate and a corresponding decrease in production costs.
It is an objective of this invention to decrease the time interval required to convert prepolymers derived from a difunctional isocyanate and a polyol to a non-deformable and demoldable state using any of the conventional aromatic diamines, particularly the class of compounds disclosed in the aforementioned U.S. Pat. No. 3,920,617.
It has now been found that this objective can be achieved using quaternary ammonium compounds as catalysts. A significant decrease in demolding time is achieved with certain prepolymers when the quaternary ammonium compound is employed in combination with a mono- or polyfunctional carboxylic acid containing from 4 to 20 carbon atoms. These acids are conventional catalysts for prior art chain-extending agents such as "MOCA".